The invention relates to a rock anchor.
As is known, when extending a cavity in a broken and difficultly-borable rock, the latter must be stabilised or solidified by inserting anchors. It is known in this connection to use so-called injection tube anchors. These consist generally of a strong-walled tube, one end of which is closed and also runs to a point and the other end carries a threaded part. The surface of the tube is provided with numerous openings. Such a tube anchor is inserted into existing boreholes whereby, on forcing in cement mortar which exudes through such openings, the space between the borehole and the tube anchor becomes filled. Since in this way the cement mortar also passes into the spaces in the rock surrounding the borehole, the surrounding rock is solidified on hardening, so that the tube anchor serves in known manner as a tie-rod. Since the tube anchor can be rammed directly into the rock only in exceptional cases, this process of stabilisation of a cavily presupposes the preliminary provision of a borehole.
The formation of boreholes, which can attain a depth of up to 12 m, therefore frequently involves difficulties in broken rock, since recovery of the tie-rods is problematical and consequently is often connected with their loss. The expenditure of time and cost arising in this way from the formation of the borehole is a considerable overhead in the construction of a tunnel, which is significant especially with borehole depths of more than 4 m, since in these cases the inserted tie-rods have to be extended. Furthermore, if the tie-rods have to be extended, the straightness of the borehole often cannot be ensured exactly with varying rock formations. The possibilities for error thus arising render difficult insertion of the rock anchor after formation of the borehole.
It is known to use drilling rods simultaneously as anchors, cement mortar being injected under pressure into the borehole, after insertion of the drilling-rod by way of the hydraulic bore in the drill-head. In these cases, spreading out of the cement mortar in the borehole is very limited, so that in practice only the tip of the drilling-rod is cemented into the surrounding rock, whereas the remainder of the drilling-rod has no connection with the rock. The deficiency in stabilisation of the rock which thus arises cannot be assessed in many cases.
From DE-PS No. 936, 082, a borer for dry borings to be used simultaneously as a rock anchor is known, whose drilling-rod is provided with a central longitudinal bore, from which transverse borings branch off and are distributed uniformly over its entire length. The axes of the borings extend from the aforementioned longitudinal bore at angles to the transverse section plane in the direction of the base of the borehole. The drilling-rod is provided on its outside with a screw-thread-shaped surface structure which is formed by a welded or soldered wire or the like, for example forged beads. The aforementioned transverse borings serve for the removal of the rock dust, which is produced during a drilling phase in the region of the drill-head and is removed via the central longitudinal bore. After completion of the boring, cement milk is injected into the borehole via these transverse bores, by which the surrounding rock is solidified and where, by means of the surface structure of the exterior of the drilling-rod, the bond between the hardened cement filling on the one hand and the drilling-rod on the other hand is improved. Should this known borer have to be used for the injection of cement mortar, this raises the problem that uniform filling of the borehole cannot be ensured sufficiently, because of the flow-resistance of the transverse bores in the direction towards the borehole outlet.
Thus the known drilling-rods used simultaneously as rock anchors have defects which concern the injection of the cement mortar into the borehole, particularly the uniform distribution of the mortar filling. However, the quality of the bonding action between the various rock or stone formations whose position is to be stabilised, achievable by the rock anchor, depends upon the latter.